in general 6l6s stay cleaner at higher volumes, have a tighter bass end and brighter highs with a less emphasized mid range, EL-34s have a big loose bass sound with a lot of midrange and a sharper treble sound they tend to break up early. basically 6L6s=American Voiced and EL-34s=British Voiced, but it should be said that the tone really has a lot to do with how the amp is designed as well.

in general 6l6s stay cleaner at higher volumes, have a tighter bass end and brighter highs with a less emphasized mid range, EL-34s have a big loose bass sound with a lot of midrange and a sharper treble sound they tend to break up early. basically 6L6s=American Voiced and EL-34s=British Voiced, but it should be said that the tone really has a lot to do with how the amp is designed as well.